The present invention generally relates to synchronism discriminating circuits, and more particularly to a synchronism discriminating circuit which accurately discriminates whether phases of two input signals are within a predetermined synchronous range, without making an erroneous discrimination.
Generally, a rotation control system for rotating a motor in synchronism with a reference signal, is designed to compare the phase of the reference signal with the phase of a signal which is obtained by detecting the rotation of the motor by use of a frequency generator, for example. The rotation of the motor is controlled responsive to an error output which is obtained as a result of the phase comparison.
However, at an initial stage of the motor rotation, for example, the rotational speed of the motor does not reach a predetermined rotational speed, and the motor does not rotate in synchronism with the reference signal. Hence, it is necessary in some cases to detect whether the rotation of the motor is pulled into synchronism with the reference signal. A synchronism discriminating circuit is employed in such cases, to discriminate whether the motor rotates in synchronism with the reference signal. By employing such a synchronism discriminating circuit, it is possible to detect an asynchronous state while the motor rotates, when the rotation of the motor becomes out of synchronism with the reference signal due to some cause.
As will be described later on in the specification in conjunction with the drawings, a conventional synchronism discriminating circuit had a disadvantage in that the circuit may erroneously discriminate that the motor is undergoing a synchronous rotation even when the motor rotates out of synchronism.